Document intended workflow with imgpkg

**Describe the problem/challenge you have**
While integration with `imgpkg` is mentioned (specifically that one can generate a `kind: ImagesLock`), the user is left to infer the intended workflow. There are details (e.g. how meta is handled, how references are relocated, explicitly that this lockfile _is_ `.imgpkg/images.yml`) that are no documented anywhere.

**Describe the solution you'd like**
Document the end-to-end workflow:

`kbld --imgpkg-lock-output=.imgpkg/images.yml ...` ==> `imgpkg copy ...` ==> `imgpkg pull` ==> `kbld -f .imgpkg/images.yml ...`

**Anything else you would like to add:**

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